Fuel cells are devices that directly convert chemical energy of reactants, i.e., fuel and oxidant, into direct current (DC) electricity. For an increasing number of applications, fuel cells are more efficient than conventional power generation, such as combustion of fossil fuel and more efficient than portable power storage, such as lithium-ion batteries.
In general, fuel cell technologies include a variety of different fuel cells, such as alkali fuel cells, polymer electrolyte fuel cells, phosphoric acid fuel cells, molten carbonate fuel cells, solid oxide fuel cells and enzyme fuel cells. Some fuel cells utilize compressed hydrogen (H2) as fuel. Compressed hydrogen is generally kept under high pressure, and is therefore difficult to handle. Furthermore, large storage tanks are typically required and cannot be made sufficiently small for consumer electronic devices. Proton exchange membrane (PEM) fuel cells use methanol (CH3OH), sodium borohydride (NaBH4), hydrocarbons (such as butane) or other fuels reformed into hydrogen fuel. Conventional reformat fuel cells require reformers and-other vaporization and auxiliary systems to convert fuel to hydrogen to react with oxidant in the fuel cell. Recent advances make reformer or reformat fuel cells promising for consumer electronic devices. Other PEM fuel cells use methanol (CH3OH) fuel directly (“direct methanol fuel cells” or DMFC). DMFC, where methanol is reacted directly with oxidant in the fuel cell, is the simplest and potentially smallest fuel cell, and also has promising power application for consumer electronic devices. Solid oxide fuel cells (SOFC) convert hydrocarbon fuels, such as butane, at high heat to produce electricity. SOFC requires relatively high temperature in the range of 1000° C. for the fuel cell reaction to occur.
The chemical reactions that produce electricity are different for each type of fuel cell. For DMFC, the chemical-electrical reaction at each electrode and the overall reaction for a direct methanol fuel cell are described as follows:
Half-reaction at the anode:CH3OH+H2O→CO2+6H++6e−
Half-reaction at the cathode:1.5O2+6H++6e−→3H2O
The overall fuel cell reaction:CH3OH+1.5O2→CO2+2H2O
Due to the migration of the hydrogen ions (H+) through the PEM from the anode to the cathode and due to the inability of the free electrons (e−) to pass through the PEM, the electrons must flow through an external circuit, thereby producing an electrical current through the external circuit. The external circuit may be used to power many useful consumer electronic devices, such as mobile or cell phones, calculators, personal digital assistants, laptop computers, and power tools, among others.
DMFC is discussed in U.S. Pat. Nos. 5,992,008 and 5,945,231, which are incorporated herein by reference in their entireties. Generally, the PEM is made from a polymer, such as Nafion® available from DuPont, which is a perfluorinated material having a thickness in the range of about 0.05 mm to about 0.50 mm, or other suitable membranes. The anode is typically made from a Teflonized carbon paper support with a thin layer of catalyst, such as platinum-ruthenium, deposited thereon. The cathode is typically a gas diffusion electrode in which platinum particles are bonded to one side of the membrane.
Another fuel cell reaction for a sodium borohydride reformer fuel cell is as follows:NaBH4 (aqueous)+2H2O→(heat or catalyst)→4(H2)+(NaBO2) (aqueous)
Half-reaction at the anode:H2→2H++2e−
Half-reaction at the cathode:2(2H++2e−)+O2→2H2OSuitable catalysts for this reaction include platinum and ruthenium, and other metals. The hydrogen fuel produced from reforming sodium borohydride is reacted in the fuel cell with an oxidant, such as O2, to create electricity (or a flow of electrons) and water byproduct. Sodium borate (NaBO2) byproduct is also produced by the reforming process. A sodium borohydride fuel cell is discussed in U.S. published patent application no. 2003/0082427, which is incorporated herein by reference.
One of the more important features for fuel cell application is fuel storage. The fuel supply should also be easily inserted into the fuel cell or the electronic device that the fuel cell powers. Additionally, the fuel supply should also be easily replaceable or refillable.
U.S. patent publication no. 2003/0082427 discloses a fuel cartridge where sodium borohydride fuel is reformed within the cartridge to form hydrogen and byproduct. However, the prior art does not disclose a fuel supply that allows in situ production of fuel or that contains reagents amenable to non-corrosive, low cost storage, or fuel supplies with the advantages and features described below.
Typically, the MEA is located inside a fuel cell which is located inside consumer electronic devices. U.S. published patent application nos. 2003/0082416 and 2003/0082426 disclose such devices. In such devices, the fuel supply is removable and stored in a cartridge. The life of the MEA is usually limited by the life of the PEM. The PEM efficiency is susceptible to various factors such as fuel flow rate, metal ion concentration in the fuel, fuel temperature, and ambient/stack temperature. When the PEM efficiency is at a sufficiently low level, the PEM has to be replaced or refurbished. Frequent servicing of the PEM is undesirable as it requires servicing the electronic device.
A need exists for a fuel cell system that allows servicing or repairing the PEM without servicing the electronic device.